He affirmed that Iran had assisted militant groups like Hezbollah and Hamas – a well-known policy, but one that Iranian leaders rarely acknowledge explicitly.

"We have intervened in anti-Israel matters, and it brought victory in the 33-day war by Hezbollah against Israel in 2006, and in the 22-day war" between Hamas and Israel in the Gaza Strip, he said.

Israel's large-scale military incursion against Hamas in 2008-09 in Gaza ended in a ceasefire, with Israel claiming to have inflicted heavy damage on the militant organisation. The war in Lebanon ended with a UN-brokered truce that sent thousands of Lebanese troops and international peacekeepers into southern Lebanon to prevent another outbreak.

"From now on, in any place, if any nation or any group confronts the Zionist regime, we will endorse and we will help. We have no fear expressing this," said Khamenei. He said Israel was a "cancerous tumour that should be cut and will be cut".

His speech followed suggestions by Israel that military strikes are an increasing possibility if sanctions fail to halt the Islamic Republic's nuclear programme.

An Israeli foreign ministry spokesman said he wasn't surprised by Khamenei's remarks. "It's the same kind of hate speech that we've been seeing from Iran for many years now," Yigal Palmor said.

Khamenei said the US would suffer defeat and lose standing in the region if Washington decided to use military force.

"Iran will not withdraw. Then what happens?" asked Khamenei. "In conclusion, the west's hegemony and threats will be discredited" in the Middle East. "The hegemony of Iran will be promoted. In fact, this will be in our service."

Western forces have recently bolstered their naval presence in the Gulf led by the aircraft carrier USS Abraham Lincoln.

Last month, Iran's navy carried out 10 days of exercises in the Gulf. The manoeuvres by the Revolutionary Guard, under the direct control of Khamenei, were announced by Iranian state media.